Biocides are an indispensable part of numerous chemical mixtures of substances and of materials in everyday use both in the industrial and the non-industrial area. The biocides prevent that said mixtures of substances and materials spoil due to the action of microorganisms and thereby become useless. At the same time, they thus also protect the health of the consumer.
Biocides are necessary for the preservation of, for instance, cleaning agents, scouring agents, detergents, polishing agents, coating agents, adhesives, plasters, paper, cardboard, textiles, leather, drilling fluids, metal-working fluids, latex emulsions, plastics, fuels and water in cooling circuits. The intended effect of the biocides imparts the substrates to which it has been admixed, a substantially longer life, both in the closed container during storage and transportation and during the use of the substrates.
3-isothiazolinones are important biocidally active substances which incorporate the most important properties of a modern biocide. Mention must be made especially of the very wide spectrum of activity against bacteria, mold fungi, yeasts and algae and of the easy degradability in the environment. Depending on the substituents in the isothiazolinone molecule, the isothiazolinones differ in their reactivity and thus also in their action and/or they differ with respect to their solubility.
The production of 3-isothiazolinones is known and can take place in various manners. In U.S. Pat. No. 3,849,430, two general methods or processes are disclosed for the preparation of 3-isothiazolinones. In accordance with the first alternative, a dithiodiamide is converted to the corresponding 3-isothiazolinone by cyclization with a halogen agent. There is thus produced, for instance, from 3,3'-dithio-N,N'-dimethyldipropionamide by reaction with sulfuryl-chloride a mixture of 5-chloro-2-methyl-3-isothiazolinone and 2-methyl-3-isothiazolinone. In accordance with the second alternative, a mercaptoamide is used instead of the dithiodiamide. In this case, one obtains, for instance by reacting N-methyl-3-mercaptopropionamide with elementary chlorine, also a mixture of the two above-mentioned 3-isothiazolinones.
For many applications, it is necessary to formulate the 3-isothiazolinone in solutions. As solvent, there is preferably employed water, but polar organic solvents, such as alcohols, are also used for this purpose. Some of these 3-isothiazolinone solutions can exhibit chemical decomposition of the 3-isothiazolinones already during their storage, i.e. prior to their intended use, which results in a reduction of the biocidal action.
In accordance with U.S. Pat. No. 3,870,795, this disadvantage can be counteracted by adding metal nitrates or metal nitrites for the chemical stabilization of the 3-isothiazolinones in formulations. It was found, however, that this type of stabilization is not sufficient in the case of some 3-isothiazolinones which are particularly susceptible to decomposition. It is, namely, possible that there occur precipitations and losses of active ingredients which can greatly impair the use of the solutions. This can be reduced by subjecting such nitrate-stabilized 3-isothiazolinone solutions to an additional stabilization. In accordance with EP-B-95907, this takes place, in the case of a mixture of 5-chloro-2-methyl-3-isothiazolinone and 2-methyl-3-isothiazolinone, by heat treatment for four hours at 95.degree. C. In connection with the beneficial effect of heat treatment at 95.degree. C. mention is also made of U.S. Pat. No. 5,068,338 and U.S. Pat. No. 5,312,827.
It has, however, been found that this heat treatment also leads to disadvantages.
On the one hand, the relatively high temperature upon the heat treatment of the 3-isothiazolinone solution has the effect that certain impurities, which were produced as by-products upon the production of the isothiazolinone and are still present in the solution, are nitrosated by the nitrate added as stabilizer to form nitrosamines. The latter, however, are carcinogenic substances which must be avoided under all circumstances. Said by-products occur primarily upon the production of the isothiazolinone from the corresponding dithiodiamide. In addition, however, the relatively high temperature of the heat treatment results in the partial decomposition of the valuable 3-isothiazolinone, regardless of which process is used, leading to the formation of colored impurities and subsequent loss of active ingredient. Thus, the quality of the 3-isothiazolinone product produced is substantially impaired.